Panorama of Salt Pond A6, taken in 2010 before the area was breached as part of the South Bay Salt Pond Restoration Project. Today, the area has accumulated sediment and looks like a natural mudflat. The image was created by stitching together 12 wide-angle frames taken by a camera at 150 feet, suspended from a kite, and is part of the Hidden Ecologies Project, http://bit.ly/NYMgAq.

Cris Benton

A pair of avocets chase a California gull that had snatched their baby from a nest at Don Edwards S. F. Bay National Wildlife Refuge.

We were on patrol. Caitlin Robinson-Nilsen, a young biologist in shades and a ponytail, steered the 4WD Explorer along a muddy levee in Fremont, Calif., and I rode shotgun, staying vigilant. She surveys snowy plovers –– adorable, six-inch, two-ounce, skittering shorebirds, with black collars and eye-patches –– as the waterbird program director for the San Francisco Bay Bird Observatory. But we weren't after plovers that April day last spring. We were cruising the South Bay's vast salt pond system in search of something larger, more dangerous. Through the windshield was a strange land of water and sky, rimmed with office parks. They're out there. They'll steal your lunch.

We were looking for California gulls. Trash birds, some say -- but that's only a partial description of their diet. They also adeptly hunt and devour the eggs and chicks of waterbirds like plovers -- many with a tenuous hold on disappearing habitat. Now, on the metropolis' coattails, the Bay's California gull population is going gangbusters. To make matters worse, 23,000 of them -- roughly half the Bay's population -- were displaced from their nesting grounds when the levees of A6, a 330-acre salt pond, were breached in 2010, returning it to the tides as part of the South Bay Salt Pond Restoration Project.

The gulls were on the loose, hunting for new homes, and no one, least of all Robinson-Nilsen, wanted them settling near plovers. It was one of many unintended consequences of the largest wetlands restoration on the West Coast. Break down one barrier, others are likely to crumble. Restoration is seldom as simple as banishing exotics and promoting natives; it's a series of trade-offs, and the odds are that not every species will come out ahead. Nor is it easy to delineate which species actually belong in an ecosystem as radically transformed by people and invasive species as the South Bay. Still, this restoration, like most, is optimistic: It hopes we can all be winners.

Gulls are good at winning, and well-suited to human sprawl; snowy plovers, not so much. Charadrius nivosus is an undeniably endearing bird, found all over the world. But it's a nervous soul, unsettled by tall structures. Telephone poles? As scary as hills. Their subtle nest scrapes and camouflaged eggs are found only on beaches, gravel river bars and sand and salt flats, right where humans like to build high-rises and vacation homes, walk dogs and drive ATVs, activities that can disturb or destroy nests. As if that weren't enough, snowies also have to contend with European beachgrass and climate change swallowing up shoreline nesting grounds. And now, an explosion of gulls.

Two subspecies of snowies exist in North America, the Cuban along the Gulf Coast, and the western, which consists of distinct inland and coastal populations. The latter -- Robinson-Nilsen's charge -- ranges from southern Washington to the tip of Baja, but most reside in the Bay Area and southward. In 1978, both the inland and coastal western snowies became species of special concern in California, but only the coastal birds were listed as threatened under the Endangered Species Act, in 1993.

The first statewide survey between 1977 and 1980 found snowies missing from 33 of 53 known coastal breeding locations. In the Bay Area, 351 birds were counted. By 1991, there were only 176. The low -- 72 -- came in 2003. Since then, the numbers rose to about 250. But this year, just 147 were spotted. The Bay Area is thought to host 5 to 10 percent of the entire coastal breeding population, almost all of them in the salt ponds. It's primo habitat: light-colored expanses to blend in while nesting; wet edges for hunting brine flies, using the run-stop-peck approach; and blessedly few people, since it's mostly off-limits to the public.

April to May is peak migration, and western sandpipers dotted every shallows. They were innocent. We passed them by. Up to a million migrating waterbirds stop in the San Francisco Bay on their journeys to their Northern breeding grounds, but Robinson-Nilsen spends her entire field season here, turning circles. Originally from Vermont, she discovered her passion for threatened piping plovers on Long Island. She studied snowies for her master's at San Jose State, and it became her dream job: "Anything to do with nesting plovers." She listens to the wind or to NPR, seldom bored. But as we zigzagged, she confessed, "I end up talking to the birds a lot."

When we reached the levee between A22 and A23 -- A for Alviso, a marsh-side community in northern San Jose -- there they were: A flock of roosting gulls, insouciantly ruffling their feathers, and ours, in the breeze. They're pretty snowy themselves, with a black ring and red spot on their yellow bill, and a four-foot wingspan. She counted them through her open window, jotting "110" in the California gull column of her datasheet. "Quite a bit less," she said.

But even one was too many: This was one of the bird observatory's "no gull zones." Earlier in the month, an irksome 700 had loafed here, right between two ponds with a handful of nesting plovers. "We were like, 'Oh, my goodness, we'll never be able to keep them off,' " Robinson-Nilsen remembered. The bird observatory began hazing them twice daily, and when we arrived, things seemed under control. "Once they lay eggs, they're protected by the Migratory Bird Act," said Robinson-Nilsen. "So we're putting in a big effort now."

We drove onto the levee, spattered with whitewash, chicken and rib bones, like the mouth of an ogre's cave. Robinson-Nilsen pointed south. "That big 'mountain' is Newby Island Landfill," she said. "Hundreds, if not thousands, of gulls feed there everyday." To the north was another landfill, making this levee an ideal midway hangout. We parked, stared the gulls down. It wasn't high noon, but close enough. On Robinson-Nilsen's command, we swung our doors wide and sprang out. She blew a silver pea whistle bought in Chinatown, and the sound carried like a battle cry. I wielded my notebook. The gulls flinched, scattering with a few resentful mews. Small, sweet payback, I thought, for all those pilfered hotdogs.

EXECUTIVE DIRECTOR
San Isabel Land Protection Trust seeks an executive director who possesses fundraising experience and an interest in land conservation. The successful candidate will be comfortable...

ASSOCIATE DESIGNER/PHOTO EDITOR
High Country News seeks a multi-talented visual journalist to join the team in rural Paonia, Colorado. Design magazine pages, find/assign great photojournalism for print and...